1. Field of the Invention
This invention relates to methods for operating a dynamic response polymer dispersed liquid crystal (PDLC) cell to increase its transmissivity and responsivity, and to PDLC light valve systems employing such methods.
2. Description of the Related Art
Photoactivated and charge-coupled device (CCD) addressed liquid crystal light valves (LCLVs) are well known, and are described for example in Margerum et al., "Reversible Ultraviolet Imaging With Liquid Crystals", Appl. Phys. Letters, Vol. 17, No. 2, 15 July 1970, pages 51-53, Efron et al., "The Silicon Liquid-Crystal Light Valve", Journal of Applied Physics, Vol. 57, No. 4, 15 February 1985, pages 1356-68, Efron et al., "A Submicron Metal Grid Mirror Liquid Crystal Light Valve for Optical Processing Applications", SPIE, Vol. 1151, 1989, pages 591-606, and Sterling et al., "Video-Rate LCLV Using an Amorphous Silicon Photoconductor", SID 90 Digest, Vol. 21, paper 17A.2, 16 May 1990. They use various types of nematic liquid crystal layers to modulate a readout light beam, which may be used in a transmissive or reflective mode of operation, depending upon the design of the LCLV and the input signal. Photoactivated LCLVs are often addressed with an input image to be amplified, such as that presented by the phosphor screen of a cathode ray tube or a scanning laser beam, particularly for dynamic modulation of a readout beam.
Nematic LCLVs operate by modulating the spatial orientation of the liquid crystals in a cell, in accordance with the input signal pattern. This often requires the use of polarizers to obtain a corresponding modulation of the readout beam. The polarizers, however, reduce the total light throughput. Also, alignment layers are needed on each side of the cell for surface alignment of the liquid crystals, thus adding to the expense of the device. The response time of the nematic liquid crystal to changes in the voltage across the cell may also be somewhat limited.
More recently, polymer dispersed liquid crystal (PDLC) films have been reported, including the use of such films in photoactive LCLVs and in active matrix LCLV projection displays. Unlike most nematic liquid crystal cells which modulate the optical polarization in response to an applied voltage, PDLCs scatter light and become transparent with an applied voltage. They have several advantages over nematic liquid crystal devices, including the elimination of surface alignment layers and polarizers, and a faster response time. However, while PDLCs exhibit a rapid response to a shift in applied voltage between fully OFF and fully ON voltage levels, their response to gray scale levels (levels not fully on or fully off) is quite slow.
LCLVs are generally operated with an alternating current applied voltage, since the liquid crystals tend to deteriorate under a DC voltage. There have been several reports of the response of PDLC-type films to square wave type voltage pulses, including the use of such films in photoactivated LCLVs and in active matrix LCLV projection displays. In Afonin et al., "Optically Controllable Transparencies Based on Structures Consisting of a Photoconductor and a Polymer-Encapsulated Nematic Liquid Crystal", Sov. Tech. Phys. Lett., Vol. 14, No. 56, January 1988, pages 56-58, a PDLC-type film was photoactivated with a ZnSe photoconductor. The photoactivated rise and decay times (with a constant bias voltage in typical LCLV operation) were 5-10 ms on-time and 1.5-3 seconds off-time; thus, the frame time (on-time plus off-time) is very slow compared to a dynamic television image frame time of less than 33 ms. The response of the PDLC-type film layer to a square voltage pulse was much faster, with rise and decay times of less than 1 ms and 15 ms, respectively, but such a pulse shape and response time is not attainable with this photoconductor.
In Macknick et al., "High Resolution Displays Using NCAP Liquid Crystals", Liquid Crystal Chemistry, Physics, and Applications, SPIE, Vol. 1080, January 1989, pages 169-173, fast response PDLC-type films were reported with a square pulse input signal of 5.3 ms. About 50% transmission was reached during the 5.3 ms pulse, and the decay time at the end of the pulse was about 2 ms; full voltage activation of the film was not shown.
In Takizawa et al., "Transmission Mode Spatial Light Modulator Using a B.sub.12 SiO.sub.20 Crystal and Polymer-Dispersed Liquid Crystal Layers", Appl. Phys. Lett., Vol. 56, No. 11, March 1990, pages 999-1001, fast photoactivated PDLC film response of 10 ms ON and 36 ms OFF was reported for a 60 ms square pulse of bright white activating light with a 30 volt bias across a photoconductor/PDLC cell. A hysteresis loop was reported when the cell was scanned with increasing and decreasing writing light intensities, but the loops were said to disappear when pulsed write light was incident on the device. Response times were reported and discussed only for square wave intensity writing light pulses.
In Kunigita et al., "A Full-Color Projection TV Using LC/Polymer Composite Light Valves", SID International Symposium Digest, May 1990, pages 227-230, a low voltage PDLC-type film was used in an active matrix display with a poly-Si thin film transistor and a storage capacitor for each pixel. Three active matrix cells were used for red, blue and green channels of full color projection TV. The response time of the PDLC-type film to a square wave voltage pulse was given for a full on-time of 35 ms and a decay ) time of 25 ms. The use of a storage capacitor at each pixel was necessary to obtain square wave voltage pulses used in this display.
In Lauer et al., "A Frame-Sequential Color-TV Projection Display", SID International Symposium Digest, May 1990, pages 534-537, a PDLC active matrix display was made with CdSe thin film transistors. The time response characteristics were fast enough for sequential three-color filtering effects at 50 Hz (6.67 ms for each color). PDLC response times were reported only for 50 volt square wave pulses of 5 ms, with the PDLC reaching a transient 60% transmission level in 5 ms of on-time, and decaying in about 2 ms, giving a relatively low light throughput in the frame time. Full projection light illumination was reported as having a large effect on the thin film transistor off-state current.
In each of the above papers, the PDLC response is described with respect to an idealized step-voltage change, or to a square wave pulse.